Given that this update will become reality , you have the following possibilities to view the subtitles on your TV. Same counts for a computer, since you seem to have problems with MKV-playback on you compuer instead of your TV.

A) For AVIs you normally have to name the *.SRT-subtitle file exactly the same way like the *.AVI file and place it in the same subfolder where the movie is stored.

B) For MKVs you normally have to mux the *.SRT-subtitle file into the MKV-container using a program package called MKVtoolnix. It contains MKVmerge. Just open the MKV file in MKVmerge, add the SRT file and merge both files to a new MKV file.

Concerning your problem:
"1. I have problems playing mkv files on my computer from an external hard drive?"

Seems like your computer does not have the neccesary codecs installed. Either download a codec-package and install it or download and install Videolan client, a player which is shipped with all neccesary codecs and which will play nearly every filetype.

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I'm having an odd issue with my 59PFL4906 TV that I can't find a solution for. When my TV spends some time powered down, it has difficulty powering on again. The remote and the power button on the TV...